Direct imaging of many biochemical processes is now practicable through the use of radiopharmaceuticals targeted towards specific disease-associated molecular targets. This was made possible by the discoveries in the field of disease related changes in cellular communication and metabolism, especially in cancer. To support these new diagnostic applications, methods for linking radioisotopes to the appropriate targeting biomolecules were required to replace the simple metal chelates and ions used previously. In the 1980s and 1990s, methods were developed for labeling biomolecules, especially monoclonal antibodies, with radionuclides such as technetium-99m (99mTc) and indium-111 (111In). In its most developed form, this typically entailed covalent attachment of a bifunctional chelator to a protein, followed by labeling with the radiometal, or even synthesis of a radiometal bifunctional chelate that was subsequently attached to the protein. As recognition grew in the 1980s that monoclonal antibodies were too large to offer ideal pharmacokinetics, focus shifted onto smaller molecules, such as antibody fragments and especially smaller peptides, targeted towards receptors present in lesions such as tumors and thrombi. Examples include radiolabeled octatreotide (selective for somatostatin receptors expressed by tumors of the endocrine system such as paragangliomas and neuroblastomas), bombesin (receptors for this peptide are expressed by small-cell lung carcinomas), and α-melanocyte stimulating hormone (expressed by melanomas).
Although the transition to smaller molecules brought with it the opportunity to use peptides produced by solid phase peptide synthesis (SPPS) rather than proteins of biological origin, the same methods were used to label them as had been used to label antibodies. These methods have several disadvantages, which are more problematic with small peptides than with large proteins. The most suitable sites for attachment of a bifunctional chelator in most peptides are the ε-amino groups of lysine residues and the N-terminus, because they are very reactive nucleophiles and form very unreactive covalent links with the chelator. If there is more than one lysine in the peptide chain, the site of modification becomes uncertain. For instance, if the peptide has two lysines, together with the N-terminus these will present three possible sites for conjugation, hence forming as many as eight products when treated with an active-ester-containing bifunctional chelator or radiolabeled bifunctional chelate. Each of these products will have a different biodistribution and different affinities for the target (some of which may have lost all target affinity) and such a mixture is not acceptable for clinical use. Moreover, one or more of the lysines may be essential to the biological activity of the peptide. A simple solution has been to incorporate the chelator, or a radiolabeled chelate or organic prosthetic group, as the last step of SPPS. This, however, has the limitation that the chelator has to be at one end of the peptide chain, which is frequently essential to the biological activity of the peptide.
The state of the art in linking radiometals to peptides encompasses a number of approaches. Some have the advantage of incorporating the metal binding sequence during SPPS, and others have the advantage of incorporating chelators that are specifically designed for the particular metal. Few, however, have both of these advantages. For example, technetium-chelating amino acid sequences such as gly-gly-cys are incorporated during SPPS or recombinant protein production, but this sequence is not ideal for its purpose, and merely represents the best that can be achieved for chelating the TcO3+ core using “standard” amino acids (i.e. those coded through tRNAs). Likewise, polyhistidine sequences, such as hexahistidine, can be incorporated during SPPS, but again they merely represent the best sequence of coded amino acids achievable for chelating the Tc(CO)3+ core. Conversely, the synthetic technetium ligand hynic (hydrazinonicotinamide) probably represents the most convenient and efficient labeling system to date for use with 99mTc, but it has so far only been used by conjugating it to a pre-formed peptide, with all the associated problems outlined above. An alternative that offers convenience of labeling is the “direct labeling” method in which antibodies and peptides containing disulfide bonds can be reduced and labeled with 99mTc or 188Re. However, the chemistry of these methods is poorly understood, and there are major stability and biological activity problems as demonstrated by the work of several groups world wide with antibodies and somatostatin analogues.
WO 2004/022106 discloses technology that incorporates the metal binding moiety during SPPS and incorporates chelators that are specifically designed for the particular metal. More specifically, WO 2004/022106 discloses metal-chelating precursors, designed to bind specific metallic radionuclides and incorporating a pendant protected (e.g. Fmoc) amino acid functionality. The chelator is attached to an amino acid before rather than after SPPS assembly of the peptide chain. A chelator-derivatized amino acid comprises: 1) an optionally protected primary or secondary amino group; 2) a carboxylic acid group; 3) a chelator group capable of binding a metallic radionuclide. Suitable chelating or metal binding groups may be chosen from several structures including but not limited to the hydrazinonicotinamide group, di- or poly-thiol groups, macrocyclic ligands incorporating amine, thioether, or phosphine donor groups, or polyaminocarboxylate groups.
As discussed above, there is a continuing need for more versatile and controlled approaches to the synthesis of metal conjugates useful, inter alia, in the diagnosis and/or treatment of diseases and conditions. The present invention focuses on novel compounds directed to these and other important uses.